Hyhridization of Echinoids. 5 



This study has been based on an examination of both living and 

 fixed material. The development of the straight-fertilized eggs was 

 followed through 6 days. Plutei were kept alive for 2 weeks, but 

 did not advance strikingly beyond the stage of development shown 

 by the 6-day larvae. The cultures were kept in finger-bowls and 

 the larvae were given a complete change of water daily, after the third 

 day, by transferring the contents of the bowls to centrifuge tubes and 

 centrifugalizing gently, thus throwing the larvae to the ends of the 

 tubes, then withdrawing the water from above the larvae and replacing 

 with fresh sea-water. The plutei treated in this manner kept in 

 better condition than those not centrifugalized, but their rate of 

 growth was slow. The general form of the larvae changed very 

 slightly, but there was some increase in the size of the skeleton. 



I was at first of the opinion that the sole cause of the slow rate 

 of growth was an inadequate supply of food, but the observations 

 of Prouho and Mortensen on developing Cidaris larvae have caused me 

 to change my opinion to the extent of believing that, even though the 

 quantity of food available was somewhat below normal, this was not 

 the only cause of what seems a slow rate of growth when compared 

 with that of other Echinoid larvae. Development in Cidaris is 

 apparently slow; these larvae may have been growing at nearly the 

 normal rate. 



Because of limited time, no attempt was made to rear the larvae to 

 metamorphosis by the use of cultures of diatoms. Material was 

 fixed in sublimate-acetic (98 c. c. saturated aqueous solution of cor- 

 rosive sublimate plus 2 c. c. glacial acetic acid), for 15 minutes. For 

 some purposes, portions of this material have been stained in toto 

 and mounted entire; for other purposes portions of the material were 

 embedded in paraffine in the usual way, cut into sections of 5 or 7 

 microns, and stained by Heidenhain's iron haematoxylin method. 

 Larvae kept for the study of skeletal rods were killed in fresh water 

 and then transferred to alkaline alcohol. 



Early Development of Cidaris Tribuloides. 



The living eggs of Cidaris tribuloides are exceedingly transparent. 

 They may be fertilized readily in the laboratory with species sperm 

 or with the sperm of other sea-urchins abundant in the region, 

 among these being Lytechinus variegatus and Tripneustes esculenta, 

 and the union of the male and female nuclei may be followed without 

 diflficulty. For straight fertilization the eggs were carefully washed 

 and inseminated within a few minutes after their removal from the 

 gonad. 



The crosses are easily made. The eggs may be activated with the 

 foreign sperm at once after their removal from the ovary, without 

 artificial aid, although as a check against the possibility of error due 



